WEDNESDAY, Sept. 19 (HealthDay News) -- The number of parents who opted out of school-required vaccines for their children because of non-medical reasons, such as religious or philosophical beliefs, increased between 2005 and 2011, according to U.S. researchers.

During this period, the rates of non-medical exemptions were higher in the states with easy opt-out policies, such as California and Maryland, and in those states that allowed philosophical, instead of only religious, exemptions.

"The more relaxed these requirements are, as we and others have shown, the easier it is to get an exemption, the higher the rates of exemptions," said Saad Omer, an infectious disease epidemiologist at Emory University in Atlanta, and lead study author.

"It is common sense to me that it should not be easier to file for an exemption than it is to get your kid vaccinated," Omer said.

Every state requires vaccines for school attendance that protect against diseases including measles, mumps and rubella (MMR), polio, chickenpox, and diphtheria, tetanus and pertussis (DTaP), according to the U.S. Centers for Disease Control and Prevention.

Religious exemptions are permitted in every state except Mississippi and West Virginia, and 20 states also accept philosophical exemptions, the CDC notes.

Previous research led by Omer found that pertussis, or whooping cough, an infection that can be deadly in babies, was more common in states that had easy exemption policies and that permitted philosophical as well as religious exemptions.

The new study, published in the Sept. 20 issue of the New England Journal of Medicine, updates an earlier study by Omer and others that looked at vaccine exemption rates among children entering kindergarten and first grade between 1991 and 2004.

During this earlier period, the researchers found that, unlike in states with easy and philosophical exemption policies, the rates did not increase in states with medium or difficult and religious-only nonmedical exemption policies.

"Vaccine hesitancy is becoming a little more widespread," Omer said.

Although the reasons behind the recent and more broad-based vaccine fear are complicated, at least two factors could be playing an important part, Omer said. Social media in the past several years, namely Facebook and Twitter, have been amplifying messages from advocacy groups and mothers about vaccines, both negative and positive. At the same time, Omer added, parents are no longer seeing firsthand the diseases that vaccines prevent, thanks to successful immunization programs.

In the current study, Omer and his colleagues compared the rates of non-medical exemptions for school-required immunizations collected by the U.S. Centers for Disease Control and Prevention for the school years between 2005-2006 and 2010-2011 with state-level exemption policies.

Even though the rates of non-medical exemptions climbed in all the states over this period, the overall rate was 2.5 times higher in states that allowed philosophical and religious exemptions compared to those that only allowed religious exemptions.

However, the rate of exemptions grew at a faster pace in states that only allowed religious exemptions over the study period, probably because their rates were so much lower to begin with, Omer said.

In addition, the non-medical exemption rate was 2.3 times higher in states with easy exemption policies compared to those with difficult policies. Whereas the non-medical exemption rate in states with easy policies grew by 13 percent each year and was 3.3 percent in 2010-2011, the annual rate increase in difficult states was 8 percent and the actual non-medical exemption rate in 2010-2011 was 1.3 percent.

These findings bolster the notion that providing fewer or more difficult avenues by which to claim non-medical exemptions means fewer parents decide to skip their child's immunizations, Omer said.

States with easier exemption policies should adopt the policies of more difficult states, where parents, for example, have to write letters explaining specifically why they want their child to be exempt and have the letters notarized, Omer added.

In California, by contrast, parents simply check a box on the school immunization form to opt out of vaccines, Omer said.

While some states with easy policies have already started to toughen them up, possibly because they have had outbreaks of vaccine-preventable diseases, other states with difficult policies are relaxing them, Omer said.

Commenting on the findings, Dr. Mark Schleiss, a professor of pediatrics at the University of Minnesota Medical School, said, "The data speak for themselves -- the more alternatives and options that are provided to decline immunizations, the greater the likelihood that immunizations will be declined."

And there are sometimes tragic results of not getting adequate vaccinations, such as the five children in Minnesota who got sick from Haemophilus influenzae type b in 2007, Schleiss said. One child died.

However, Schleiss thinks that the answer is to address public perception of vaccines instead of rewriting policies. "It has to come from our political leaders. I think they need to articulate [the importance of getting vaccinated]," he said.

Schleiss said that spending time talking openly with parents about the safety of vaccines -- and how reports in the early 1990s about the relationship with autism were completely fraudulent -- usually helps parents understand that they should get their children vaccinated.